Available bandwidth on transmission systems is a valuable commodity whose value continues to increase as more and more users and applications crowd the spectrum. As a result, maximizing the use of available bandwidth is an important concern for the industry. To date, systems have not adequately provided for user flexibility in conjunction with maximum use of available bandwidth.
Current technology permits modulation of a binary base band signal into a radio frequency (RF) signal for transmission and demodulation back into base band. As shown in FIG. 1, the base band signal 1 enters the modulator 3 and is converted into RF for transmission and receipt over antennas 5, 7. Demodulator 9 converts the received signal back into a base band signal 11. This transmission scheme is known as single channel per carrier (SCPC).
Modulators convert base band signals from binary into the frequency spectrum through a variety of modulation techniques. Common modulation techniques include binary phase shift keying (BPSK) and quadraphase shift keying (QPSK). BPSK has a conversion rate of approximately 1 kilohertz (KHZ) per 1 kilobit (KB). QPSK has a conversion rate of approximately 0.5 KHZ per 1 KB. Accordingly, QPSK is more efficient in that nearly twice as many bits of information can be transmitted over a similar frequency bandwidth. However, noise tradeoffs exist as data conversion rates increase. This limits the effectiveness of increasing bandwidth usage through modulation techniques with even higher data conversion rates.
As shown in FIG. 2, SCPC systems generate a separate RF carrier signal 13, 15 for each base band input signal 14, 16. FIG. 3 shows a plot of power versus frequency for the carrier signals 13, 15 wherein each signal occupies a separate center frequency 17, 19 with a separate bandwidth 21, 23. Since each channel--with a separate carrier--occupies different space on the frequency spectrum, such SCPC systems are inherently inefficient for multi-channeled systems.
Referring to FIG. 4, to maximize efficiency, the space 25 between each carrier signal must be minimized. However, as shown in FIG. 5, if this space is minimized too much, then the edges, or "skirts" 27, of the carrier signals overlap and interfere with each other. This can lead to erroneous and noisy demodulation of the RF signal. Alternatively, as shown in FIG. 6, the skirts 27 can be truncated via filtering, but then part of the original carrier signal has been excluded. This again could appear as errors or noise upon demodulation.
Current technology also includes multiple channel per carrier (MCPC) systems as shown in FIG. 7. With this system, multiple binary base band signals (or channels) 31, 33 are multiplexed via a multiplexor 35 and then fed into a modulator 37. The transmitted RF signal is then demodulated (via 39) and demultiplexed (via 41) into its component base band signals 43, 45. As shown by FIGS. 8(a) and 8(b), separate carriers--that might be produced by signals 31, 33 in an SCPC system--would have the potentially noisy skirt overlap 49, and a collective bandwidth 47. By multiplexing the signals together, the resulting RF signal shown in FIG. 8(b) would have a comparable bandwidth 51 and yet carry more information (e.g. up to 20% more bits), with less noise, due to more efficient use of the carrier signal across the corresponding bandwidth 51. Accordingly, MCPC systems are inherently more efficient than SCPC systems.
While MCPC systems might be more efficient, they are often used in very inefficient ways due to the inflexibility of existing transmission systems. For instance, to gain the benefits of multiplexing two (or more) signals together, information must often be transported or transmitted back to the facility where the MCPC multiplexing and transmission ultimately occurs. This practice is known as "backhauling" information. Referring to FIG. 9(a) an SCPC system 56 is shown with the resulting plot of carrier signal 57. FIG. 9(b) shows an MCPC system 58 which multiplexes the signal 57 with the backhauled signal 55 to produce the resulting MCPC carrier signal 59. FIG. 10 demonstrates the relative inefficiency of backhauling; not only is the bandwidth of signal 59 being used on the frequency spectrum, the bandwidth of signal 55 is also being used. Hence, the use of multiple carriers to create an MCPC signal is relatively inefficient, particularly when backhauling is employed, because more frequency bandwidth is ultimately used than with the MCPC system alone.
The applicant has recognized the need for a multiple channel multiple carrier system (MCMC) which is more flexible and allows users of all sizes to access the system. Multiple carriers, each carrying multiple channels, can be spread out over the available frequency spectrum, thus maximizing bandwidth usage. Each carrier will carry control header information which will allow location and access to all possible channels spread out over all possible carriers.
Existing transmission systems transport audio and video data in satellite and cable TV applications. FIG. 23 illustrates an exemplary audio/video transmission system including an audio/video encoder 400 which communicates with a statistical remultiplexor 402 which in turn communicates with a modulator 404. The encoder 400 receives audio and video signals along input lines 401 and 403 and outputs encoded packets of audio and video data along lines 406 and 408, respectively. The statistical remultiplexor 402 combines the audio and video data packets (according to the format illustrated in FIG. 25) and outputs same as an aggregate bitstream along line 412. The aggregate bitstream is transmitted to a remote destination via antenna 418 by the modulator 404. Feedback lines 410 and 414 are provided to maintain a desired timing relation between the data transmission rates of the encoder 400, remultiplexor 402 and transmit module 404.
The transmitted bitstream is received by a demodulator and the audio and video data packets are demultiplexed and decoded into separate audio and video data streams. These decoded data streams are processed and displayed to end viewers. One such demultiplexor and decoding system has been proposed LSI Logic Corporation of California (Model No. L64007 MPEG-2 Transport Demultiplexor). The system proposed by LSI Logic complies with the international standard ISO/IEC 13818-1 MPEG-2 systems specification. As shown in FIG. 25, the aggregate bitstream 450 is composed of plurality of data packets 452, each of which includes a data section 454 and a "presentation time stamp" 456 (explained below in more detail). As shown in FIG. 25, the statistical multiplexor 402 (FIG. 23) intersperses the audio and video packets in a non-uniform manner. By way of example, a single audio packet 458 may be followed by two video packets 460 and 462, which are followed by alternating audio and video packets 464-472. The statistical remultiplexor 402 controls the order in which the audio and video packets 458-472 are combined.
The presentation time stamps 456 are provided within each data packet 452 by the encoder 400 to enable synchronization and realignment, at the downstream end, between the audio and video signals. Each time stamp 456 represents a timing offset, with respect to a reference time Tr, at which corresponding audio or video packet is to be played/displayed.
However, conventional audio/video encoding and decoding have met with limited success. These existing systems have been unable to combine multiple audio and video signals into a single aggregate bitstream in an optimal manner. As explained above, conventional systems utilize statistical remultiplexors 402 to combine audio and video packets.
FIG. 26 illustrates an exemplary aggregate bitstream produced by a statistical remultiplexor which receives input signals from multiple audio and video encoders. In the example of FIG. 26, it is assumed that three audio and video encoders are utilized, denoted encoders A, B and C. According to the conventional technique, the statistical remultiplexor combines audio and video packets from these multiple encoders A-C in a statistical fashion (as shown in FIG. 26). Thus, packets pertaining to a particular video encoder or a particular audio encoder may be separated by several packets from different encoders. Time stamps generated by a single encoder represent an offset which is reset to a new reference time at time intervals of a duration only sufficient to account for the maximum delay between audio and video data packets for a single encoder. Hence, packets statistically multiplexed from two or more encoders exceed the time interval between reference times. Accordingly, the statistical remultiplexor must adjust each presentation time stamp to account for the increased delay due to the use of multiple encoders. These modified time stamps are denoted by reference numerals 480-494.
However, the foregoing statistical multiplexing process is excessively complex, slow and undesirable.